In designing components for vehicle combustion engines, the little space available is a strong limitation which very often requires redesigning the configuration and arrangement of some of its components.
The increase in the number and type of components that are gradually incorporated in the engines and must be placed in the same space make this limitation worse. This is the case of EGR systems which recirculate part of the exhaust gases towards the inlet for injecting gas without oxygen into the combustion chamber and thus reduce the percentage of nitrogen oxides generated.
The gas that exits after combustion is at a high temperature such that, before reintroducing part of this gas into the inlet, it is necessary to reduce its temperature. The temperature of this gas is reduced by using a heat exchanger. The heat exchanger directs two flows, a flow of the gas to be cooled and a flow of a cooling fluid which removes heat from the gas to reduce its temperature.
The gas flow circulates through a core of ducts surrounded by the cooling fluid. The cooling fluid flows between the core of tubes and the outer casing of the exchanger. Both flows have their inlets or outlets duly connected, communicating the exchanger with the ducts of the engine distributing both flows by means of suitable connections.
The core of ducts through which the gas circulates undergoes great temperature changes as it goes from being at rest to operating, cooling the hot gas. These temperature changes cause the expansion or shrinkage of the core of ducts. This expansion occurs in the casing to a lesser degree since this is mainly in contact with the cooling fluid. The hot gas mass flow to be cooled is high, hence the dimensions and particularly the length of the core of ducts are significant and its expansion may cause great increases in length. The difference in temperatures in the casing and in the core of ducts gives rise to different degrees of expansion. If the ends of both components were fixed, it would give rise to very high stresses and to the breakage of the part.
A solution which is commonly applied is fixing one of the ends of the core and the casing, whereas the other end of both components is fixed by means of a floating attachment allowing the relative longitudinal displacement of one with respect to the other. The fixed attachment is normally done through a flange. The weight of the core, the casing and the cooling fluid housed between the casing and the core is very high, hence the flange is a rather bulky component in order to be able to offer sufficient structural strength.
The other end of the exchanger has a floating attachment between the core and the casing, the core of ducts converging into a manifold which is extended according to a bushing with a determined diameter which is fitted inside another larger bushing arranged in the casing. O-ring gaskets preventing the cooling fluid from exiting are placed between both bushings. The O-ring gaskets limit the movements in transverse directions up to a certain point. The longitudinal direction is not impeded and the axial or longitudinal displacement between both bushings is possible as the result of the sliding of the O-ring gaskets.
The inner bushing whereby the manifold of the core is prolonged is in direct contact with the gas. The O-ring gaskets are made of an elastomer which does not withstand temperatures as high as metal does. O-ring gaskets typically degrade above 180° C. This limitation implies that in the state of the art the floating end of the heat exchanger corresponds with the exit of the cooled gas where the temperature of the gas is lower.
Certain arrangements of components in the cavity where the engine is located prevent housing the volume required for the fixed fixing of the exchanger, which fixing is not floating, and this cannot be exchanged with the floating area since exchanging the cold inlet with the hot inlet at the floating end (where the volume is less) would give rise to the degradation of the O-ring gaskets due to excessive temperature.
The present invention solves this technical problem by modifying the configuration in the hot gas inlet allowing the entrance of hot gas at the end where the core of ducts of the exchanger is a floating core.